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Skeletal muscles are attached (with some exceptions, such as the muscles of the tongue and pharynx) to the skeleton by means of tendons, usually in pairs that pull in opposite directions, e.g., the biceps (flexor) and triceps (extensor) that move the forearm at the elbow. The means by which all types of muscles contract is thought to be generally the same, although muscles are classified as phasic, or fast twitch, and tonic, or slow twitch, to differentiate between the various lengths of time a muscle may require to move in response to stimulation. Striated muscle is usually considered phasic, while cardiac and smooth muscle are thought to be tonic.
Perhaps because its action is most varied, striated muscle has been studied most extensively. This type of muscle is composed of numerous cylindrically shaped bundles of cells, each enclosed in a sheath called the sarcolemma. Each muscle fiber contains several hundred to several thousand tightly packed strands called myofibrils that consist of alternating filaments of the protein substances actin and myosin. Actin and myosin interact before muscle contraction, forming the contractile material actomyosin.
The energy required for muscle contraction comes from the breakdown of adenosine triphosphate (ATP), a substance that is present in the cells and is formed during cellular respiration. A muscle fiber is stimulated to contract by electrical impulses from the nervous system. The point of contact between nerve and muscle is the neuromuscular junction, where the chemical substance acetylcholine is secreted, initiating the changes that cause the muscle to contract. During resting states, some of the fibers in the musculature are maintained in a state of partial contraction, known as muscle tone. This permits muscles to contract quickly when stimulated without having to overcome the inertia of total relaxation.
The tissue in the body in which cellular contractility has become most apparent. Almost all forms of protoplasm exhibit some degree of contractility, but in muscle fibers specialization has led to the preeminence of this property. In vertebrates three major types of muscle are recognized: smooth, cardiac, and skeletal.
Smooth muscle, also designated visceral and sometimes involuntary, is the simplest type. These muscles consist of elongated fusiform cells which contain a central oval nucleus. The size of such fibers varies greatly, from a few micrometers up to 0.02 in. (0.5 mm) in length. These fibers contract relatively slowly and have the ability to maintain contraction for a long time. Smooth muscle forms the major contractile elements of the viscera, especially those of the respiratory and digestive tracts, and the blood vessels. Smooth muscle fibers in the skin regulate heat loss from the body. Those in the walls of various ducts and tubes in the body act to move the contents to their destinations, as in the biliary system, ureters, and reproductive tubes.
Smooth muscle is usually arranged in sheets or layers, commonly oriented in different directions. The major physiological properties of these muscles are their intrinsic ability to contract spontaneously and their dual regulation by the autonomic nerves of the sympathetic and parasympathetic systems. See Autonomic nervous system
Cardiac muscle has many properties in common with smooth muscle; for example, it is innervated by the autonomic system and retains the ability to contract spontaneously. Presumably, cardiac muscle evolved as a specialized type from the general smooth muscle of the circulatory vessels. Its rhythmic contraction begins early in embryonic development and continues until death. Variations in the rate of contraction are induced by autonomic regulation and by many other local and systemic factors.
The cardiac fiber, like smooth muscle, has a central nucleus, but the cell is elongated and not symmetrical. It is a syncytium, a multinuclear cell or a multicellular structure without cell walls. Histologically, cardiac muscle has cross-striations very similar to those of skeletal muscle, and dense transverse bands, the intercalated disks, which occur at short intervals. See Heart (vertebrate)
Skeletal muscle is also called striated, somatic, and voluntary muscle, depending on whether the description is based on the appearance, the location, or the innervation. The individual cells or fibers are distinct from one another and vary greatly in size from over 6 in. (15 cm) in length to less than 0.04 in. (1 mm). These fibers do not ordinarily branch, and they are surrounded by a complex membrane, the sarcolemma. Within each fiber are many nuclei; thus it is actually a syncytium formed by the fusion of many precursor cells.
The transverse striations of skeletal muscle form a characteristic pattern of light and dark bands within which are narrower bands. These bands are dependent upon the arrangement of the two sets of sliding filaments and the connections between them. See Muscle proteins, Muscular system
the body tissue of the skeletal and visceral musculatures. Muscles enable animals and man to perform very important physiological functions, such as movement of the body or its individual parts, blood circulation, respiration, passing of chyme through the digestive organs, maintenance of vascular tonus, and excretion.
The contractile function of all types of muscles is due to the transformation of chemical energy from certain biochemical processes into mechanical work. This transformation occurs within the muscle fibers. However, the contraction of skeletal and visceral muscles is only a particular case of a more general phenomenon—the mechanochemical activity of living structures. The most varied manifestations of this activity, for example, the contraction of the tail of a spermatozoon, the movement of cilia in infusorians, the disjunction of chromosomes during mitosis, and the injection of phage DNA into bacteria, appear to be based on the same molecular mechanism. This common mechanism involves a change in the conformation or the relative position of the fibrillar structures in contractile proteins.
Classification. Morphologists distinguish two main types of muscles: striated and smooth. Striated muscles include the entire skeletal musculature, which makes voluntary movement possible in vertebrate animals and in man; muscles of the tongue and upper third of the esophagus; heart muscle, or myocardium, which has a unique protein composition and contractile nature; and muscles of arthropods and some other invertebrates. Smooth muscles make up most of the musculature of invertebrates. In animals and man the muscular layers of the viscera and of the walls of blood vessels are also made up of smooth muscles. These muscular layers take part in the most important physiological functions. Some histologists distinguish a third type of muscle in invertebrates, muscle with double oblique striation.
STRUCTURAL ELEMENTS. All types of muscles consist of muscle fibers. Striated muscle fibers in skeletal muscles form bundles joined together by layers of connective tissue. The ends of muscle fibers are intertwined with tendon fibers, and through this combination muscular tension is transmitted to the skeletal bones. Striated muscle fibers are giant, polynucleated cells ranging from 10 to 100 μ in diameter. They are frequently as long as the muscles themselves, reaching a length of 12 cm in some human muscles. The fiber is covered with an elastic sheath, or sarcolemma. The cell is filled with a sarcoplasm that contains such organelles as mitochondria, ribosomes, the tubules and vacuoles of the sarcoplasmic reticulum and of the T-system, and various inclusion bodies.
The sarcoplasm usually contains bundles of numerous threadlike structures, myofibrils, which are also cross-striated, like the muscles of which they are a part. Myofibrils are from 0.5 μ to several microns thick. Every myofibril is divided into several hundred segments, 2.5 to 3 μ in length, called sarcomeres. Every sarcomere consists, in turn, of alternating bands that differ in optical density and impart to the myofibrils and to muscle fiber as a whole a characteristic striation that can be clearly seen under a phase-contrast microscope. The darker bands are bire-fringent and are called anisotropic, or A, bands. The lighter bands do not have this capacity and are called isotropic, or I, bands. The middle of the A band is occupied by a zone of weaker birefringence, the H zone. The I band is divided into the two equal halves of the dark Z membrane, which separates one sarcomere from another. Every sarcomere has two types of filaments, which consist of the muscle proteins thick myosin and thin actin.
Smooth muscle fibers have a somewhat different structure. They are spindle-shaped, mononuclear cells lacking cross-striations. They are usually 50–250 μ long and 4–8 μ wide. Uterine smooth muscle fibers are 500 m μ long. Smooth muscle myofilaments are usually not combined into separate myofibrils but are arranged along the length of the fiber in the form of numerous single actin filaments. There is no ordered system of myosin filaments in smooth muscle cells. Tropomyosin A fibers in the smooth musculature of mollusks appear to play a major role in performing the obturator function (closing of the shell).
CHEMICAL COMPOSITION. The chemical composition of muscles varies with the species, with the age of the animal, with the type and the functional condition of the muscle, and with some other factors. The principal constituents of human and animal striated muscles are presented in Table 1.
|Table 1. Chemical components of striated muscle (in percent of total wet weight of muscle)|
|Solid substances ...............||20–28|
|Creatine, phosphocreatine ...............||0.2–0.55|
|Free amino acids ...............||0.1–0.7|
|Lactic acid ...............||0.01–0.02|
On the average, water constitutes 75 percent of the wet weight of muscle. Proteins account for most of the solid mass. A distinction is made between the myofibrillar contractile proteins (myosin and actin and their complex—actomyosin—tropomyosin, α and β actins, troponin, and others) and the sarcoplasmic proteins (globulin X, myogens, respiratory pigments—such as myoglobin—nucleoproteins, and enzymes that participate in the metabolic processes in muscle). The extractive compounds that participate in metabolism and perform the contractile function of muscle are the most important of the remaining compounds in muscle fiber. These include ATP, phosphocreatine, carnosine, and anserine; phospholipides, which play a major role in metabolism and in the formation of cellular microstructures; nitrogen-free substances, for example, glycogen and its decomposition products (glucose, lactic acid, and so forth), neutral fats, and cholesterol; and finally, salts of sodium, potassium, calcium, and magnesium. Smooth muscles differ significantly in chemical composition from striated muscles, having a lower content of the contractile protein actomyosin and of high-energy compounds and dipeptides.
FUNCTIONAL CHARACTERISTICS OF STRIATED MUSCLES. Striated muscles are richly supplied with the nerves by which muscular activity is regulated from the nerve centers. The most important are the motor nerves, which conduct impulses to the muscles, causing them to become excited and to contract; the sensory nerves, along which information about the condition of the muscles reaches the nerve centers; and the adaptatotrophic fibers of the sympathetic nervous system, which act on the metabolism and slow the onset of muscle fatigue.
The combination of a motor nerve and the group of muscle fibers that it innervates is called the motor unit. Each branch of a motor nerve in a motor unit extends to a separate muscle fiber. All the muscle fibers that constitute such a unit contract almost simultaneously when they are excited. The nerve impulse causes a mediator, acetylcholine, to be released at the end of the motor nerve. Acetylcholine then reacts with the choline receptor at the postsynaptic membrane. This increases the permeability of the membrane to sodium and potassium ions, causing the membrane to become depolarized: a postsynaptic potential appears. A wave of electronegativity then arises in adjacent portions of the skeletal muscle fiber membrane and is propagated along the muscle fiber, usually at the rate of several meters per second.
The elastic properties of muscles change as a result of excitation. If the points of attachment of the muscle are not rigidly fixed, the muscle contracts, performing mechanical work. If the points of attachment of the muscle are fixed, tension develops in the muscle. A latent period ensues between the origin of excitation and the appearance of a wave of contraction or tension. Muscular contraction is accompanied by a release of heat that continues for some time even after the muscle relaxes.
The muscles of mammals and man can consist of slow (red) muscle fibers, containing the respiratory pigment myoglobin, and rapid (white) fibers, containing no myoglobin. Rapid and slow fibers differ from one another both in the rate of conduction of the contractile wave and in the wave’s duration. In mammals the duration of the wave of contraction in slow fibers is five times as great as in rapid fibers, but the rate of conduction is only half as great as in rapid fibers.
Almost all skeletal muscles are of a mixed type, that is, they contain both rapid and slow fibers. Either single (or phasic) contraction of the muscle fibers or tetanic (or prolonged) contraction can arise, depending on the nature of the stimulus. Tetany occurs when a series of stimulations reaches a muscle at such a rate that each successive stimulation still leaves the muscle in a state of contraction, causing a superimposition of the contractile waves. N. E. Vbedenskii discovered that increasing the rate of stimulation intensifies the tetany, but only to a certain limit, which he called the “optimum.” Further increases in the rate of stimulation diminishes tetanic contraction to the “pessimum.” The onset of tetany is important in the contraction of slow muscle fibers. In muscles with a predominance of rapid fibers, maximum contraction usually results from the superimposition of contractions from all those motor units that are simultaneously active. To accomplish this, the nerve impulses usually reach these motor units asynchronously.
Striated muscles also contain a third type of fiber, purely tonic fibers, which are especially well represented in the muscles of amphibians and reptiles. Tonic fibers help to maintain a continuous muscle tonus. Tonic contractions are slowly developing, coordinated contractions, capable of persisting a long time without a significant energy loss. Muscles in tonic contraction manifest a continuous resistance to any external forces that are applied toward dilating a muscular organ. Tonic fibers react to a nerve impulse with a contractile wave only at the site of stimulation. Nevertheless, owing to the large number of motor endplates—stimulation sites—a tonic fiber can still become excited and completely contract. Such fibers contract so slowly that even at very low frequencies of stimulation, individual waves of contraction superimpose and merge to produce a single, prolonged contraction. The prolonged resistance of tonic fibers and slow phasic fibers to a tensile force is ensured not only by the contractile function of the muscle proteins but also by increased viscosity of the proteins.
The contractile ability of a muscle is expressed in terms of the muscle’s absolute strength, the ratio of a muscle’s mass to the area of its cross-section taken at the plane perpendicular to the fibers. Absolute strength is expressed in kilograms per centimeter squared (kg/cm2). For example, the absolute strength of the human biceps is 11.4 kg/cm2, and that of the gastrocnemius, 5.9 kg/cm2.
Systematic exercise of muscles increases their mass, strength, and efficiency. However, excessive work results in fatigue, that is, loss of muscular efficiency. Inactivity causes muscles to atrophy.
FUNCTIONAL CHARACTERISTICS OF SMOOTH MUSCLES. Smooth muscles of the internal organs differ significantly from skeletal muscles in the manner of innervation, excitation, and contraction. The waves of excitation and contraction proceed very slowly in smooth muscles. In such muscles the development of a continuous muscle tonus is related, as in tonic skeletal fibers, to the slow rate of propagation of the contractile waves, which merge with one another even after infrequent rhythmic stimulation. The phenomenon of automatism (activity not caused by the entry of nerve impulses from the central nervous system into the muscle) is also characteristic of smooth muscles. Both the nerve cells that innervate smooth muscle and the smooth-muscle cells themselves have been found to be capable of spontaneous—independent of central nervous system stimulation—rhythmic excitation and contraction.
Smooth muscles in vertebrates are unique not only in their innervation and histological structure but also in their chemical composition. They have a lower content of the contractile protein actomyosin; fewer high-energy compounds, particularly ATP; a low ATPase activity in the myosin fraction; and a water-soluble variety of actomyosin, called tonoactomyosin.
Of great importance to the organism is the ability of smooth muscles to change length without increasing the tension exerted. Such a situation arises, for example, during the filling of hollow organs, such as the urinary bladder and stomach.
I. I. IVANOV
Skeletal muscles in man. Skeletal muscles in man, which differ from one another in shape, size, and position, constitute about 40 percent of the body mass. Upon contracting, the muscle can shorten to 60 percent of its length. The longer the muscle (the longest muscle, the sartorius, is 50 cm long), the greater its range of movements. Contraction of a dome-shaped muscle, for example, the diaphragm, results in flattening, while contraction of a ring-shaped muscle, for example, a sphincter, results in constriction or closure of the opening that the muscle surrounds. A radial muscle, on the other hand, widens the opening when it contracts. The contraction of muscles that are located between bony prominences and the skin changes the shape of the skin surface.
All skeletal, or somatic, muscles can be classified according to their location into muscles of the head (these include the facial muscles and the masseter muscles controlling the lower jaw) and muscles of the neck, the trunk, and the extremities. Since truncal muscles cover the chest and form the walls to the abdominal cavity, they are divided into thoracic, abdominal, and spinal muscles. Muscles of the extremities are classified according to which segment of the skeleton they are associated with. In the upper extremities there are the muscles of the shoulder girdle, of the shoulder, of the forearm, and of the hand; in the lower extremities there are the muscles of the pelvic girdle, of the hip, of the shin, and of the foot.
In man there are about 500 muscles attached to the skeleton. Some of them are large, for example, the quadriceps femoris, while others are small, for example, the short muscles of the back. Work that involves several muscles is performed synergistically, although some functional muscle groups work antagonistically when carrying out certain movements. For example, the biceps and the brachialis muscles in the front of the forearm flex the forearm at the elbow joint, while the triceps brachii, located in back, serves to extend the forearm.
Both simple and complex movements occur in the spheroid articulations. For example, the hip is flexed at the hip joint by the iliopsoas and extended by the gluteus maximus. The hip is abducted by the gluteus minimus and the gluteus medius and adducted by five muscles of the medial group of the hip. The hip joint is also surrounded by muscles that rotate the hip laterally and medially.
The most powerful muscles are those of the trunk. They include the muscles of the back, which keep the trunk erect, and the abdominal muscles, which constitute an unusual formation in man, the prelum abdominale. In the course of evolution, the muscles of the lower extremities in man have become stronger on account of the vertical position of the body. They support the body as well as participate in locomotion. The muscles of the upper extremities, conversely, have become more dextrous in order to guarantee the execution of rapid and precise movements.
On the basis of physical location and functional activity, modern science also classifies muscles according to the following group: the muscle group that controls movement in the trunk, head, and neck; the muscle group that controls movement of the shoulder girdle and the free upper extremity; and the muscles of the lower extremity. Smaller divisions are distinguished within these groups.
V. V. KUPRHANOV
Pathology of muscles. The impairment of contractility and the development of a prolonged muscle tonus are observed in the following disturbances, to name a few: in hypertension, myocardial infarction, and myodystrophy; in atonia of the uterus, intestine, and urinary bladder; in some forms of paralysis, for example, after recovery from poliomyelitis. Pathological changes in the functioning of muscular organs may result from disturbances in nervous or humoral regulation, from injuries to any part of the muscles (for example, in myocardial infarction), or from changes at the cellular and subcellular levels. Subcellular and cellular disturbances can involve a change in the contractile protein substrate or a change in metabolism. Metabolic changes usually occur within the enzymatic system that is concerned with the regeneration of high-energy compounds, especially ATP. Subcellular and cellular changes may be caused by the insufficient production of muscle proteins that follows the impairment of messenger RNA synthesis. Such impairment results in congenital defects in the structure of the chromosomal DNA. This last group of diseases is therefore considered hereditary.
The sarcoplasmic proteins in skeletal and smooth muscles are of interest not only because they participate in the development of the viscous aftereffect but also because many of them are enzymatically active and take part in the cell’s metabolism. When muscular organs are injured, as in myocardial infarction, or when the permeability of the surface membranes of muscle fibers is impaired, enzymes such as creatine kinase, lactate dehydrogenase, aldolase, and transaminase may escape into the blood. Thus, in certain diseases, such as myocardial infarctions and myopathies, it is of considerable clinical interest to determine the activity of these enzymes in the plasma.
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I. I. IVANOV